The invention refers to a drive circuit for two switching converter stages in a voltage converter.
Switching converters with different topologies are used for supplying a specified voltage to loads in a familiar adequate manner. The concept of connecting multiple switching converters in parallel in order to provide a large output current for a comparably low output voltage (as required for microprocessors in computers, for example) is well known. Voltage converters with switching converter stages connected in parallel are described, for example, in the publications GB 2 012 501 A or EP 1 079 508 A2. Switching converters with multiple converter stages of this nature are known as multi phase converters.
FIG. 1 illustrates a circuit diagram of a switching converter with two switching converter stages 10, 20. The two converter stages are designed in the example as buck converters, each with input terminals 11, 12, 21, 22 for applying an input voltage Vin, and output terminals 13, 14, 23, 24 for providing a regulated output voltage Vout for a load 2, depicted by dashes. The two converter stages 10, 20 are connected in parallel by connecting the first output terminals 13, 23 in a conductive manner, and connecting the second output terminals 14, 24—which are connected in the example to a reference potential GND—in a conductive manner.
Each of the two converter stages has an LC element with an inductor 16, 26 and a capacitor 17, 27, along with a first switch 15, 25 for clocked application of the input voltage Vin to the LC element. The common output voltage Vout of the converter stages 10, 20 is applied over the capacitors 17, 27. Each of the converter stages has a second switch 18, 28, which is driven complementary to the corresponding first switch 15, 25 by means of an inverter 19, 29, and which acts as a free-running element for the inductor 16, 26 with the first switch 15, 25 open.
The switches 15, 25 in the two converter stages are driven in each case by a pulse width-modulated drive signal PWM1′, PWM2′. The power input of each of the converter stages 10, 20 varies with the duty cycle of the corresponding pulse width-modulated drive signal PWM1′, PWM2′. With reference to FIG. 2, the duty cycle is determined by the quotient obtained from the activation period Ton1, Ton2 of the corresponding signal PWM1′, PWM2′ and its period Tp1, Tp2.
As described in Tarter: “Solid-State Power Conversion Handbook”, page 362, John Wiley & Sons, 1993, ISBN 0-471-57243-8, it is an established practice to compare a control or feedback signal that varies with the output voltage to a saw-tooth signal in order to provide a pulse width-modulated drive signal for controlling the power input in a switching converter. A control signal of this kind obtained from the output voltage Vout using a voltage divider R1, R2 and a control amplifier or error amplifier 51 is marked by Vfb in FIG. 1.
In order, on the one hand, to minimize rippling of the output voltage Vout and, on the other hand, to place as distributed a load as possible on an input voltage source for supplying the input voltage Vin, it is beneficial if the switches 15, 25 in the individual converter stages 10, 20 are driven as conducting with a time delay between them. With reference to FIG. 2, this can be achieved by providing two saw-tooth signals Vr1, Vr2 with a phase shift of 180° between each signal for comparison with the feedback signal.
A basic objective when employing multiple converter stages connected in parallel is to drive the individual converter stages in such a way that each stage contributes in equal measure to supplying the load—i.e., so that they provide at least approximately equal output currents.
Assuming that the components of each of the converter stages 10, 20, particularly the LC elements, are identically dimensioned, it is crucial for equal current loading of the two converter stages 10, 20 that the pulse width-modulated drive signals PWM1′, PWM2′ have a large degree of similarity with regard to their duty cycles, as otherwise, if the duty cycle of one converter stage differs even slightly from the duty cycle of the other converter stage, the output currents of the two stages differ greatly. It can be mathematically demonstrated for a typical load on the voltage converter with a supply voltage of 2V and a current rating of 10 A that the difference in the output currents of the two converters 10, 20 is about 20% if the duty cycles of the two converters differ by just 1%, assuming a turn-on resistance of 10 mΩ for the two converters.
If the two converter stages 10, 20 are driven using phase shifting, the setting of an identical duty cycle for the two converter stages 10, 20 makes considerable demands when matching the two saw-tooth signals Vr1, Vr2.
Accordingly, it would be advantageous to provide a drive circuit for two switching converter stages in a voltage converter to generate two phase-shifted pulse width-modulated drive signals with identical duty cycles.